The invention relates to a method for identifying cable faults at the terminals of a broadband lambda probe comprising a Nernst cell and a pump cell in the exhaust gas duct of an internal combustion engine, wherein the broadband lambda probe has a reference electrode terminal RE, an internal pump electrode terminal IPE and an external pump electrode terminal APE, wherein, via a control unit, a pump current is applied to the broadband lambda probe by means of a pump current source SQ and a pulsed reference pump current is applied to the broadband lambda probe by means of a reference current source SQr, wherein the pump current source SQ and the reference current source SQr are mutually connected to the terminals of the broadband lambda probe, a grounding resistor RGND and a reference voltage source URef with the aid of a first switching matrix and wherein the first switching matrix enables at least the switching connections
Z_1: RGND with IPE and APE with SQ
Z_2: RGND with APE and IPE with SQ
Z_0: IPE with VM
Z_Rie: RE with SQr and IPE with VM.
The invention furthermore relates to a control unit for operating a broadband lambda probe comprising a Nernst cell and a pump cell in the exhaust gas duct of an internal combustion engine and for detecting information about the operating state of the broadband lambda probe, wherein the broadband lambda probe has as terminals a reference electrode terminal RE, an internal pump electrode terminal IPE and an external pump electrode terminal APE, wherein the control unit is connected to the terminals of the broadband lambda probe and a grounding resistor RGND and a calibration resistor Rea wherein the control unit has a first switching matrix for mutually connecting a pump current source SQ and a reference current source SQr to the terminals of the broadband lambda probe, the grounding resistor RGND, the calibration resistor Rcal and a reference voltage source URef, wherein the control unit has a second switching matrix for mutually connecting the terminals of the broadband lambda probe, the grounding resistor RGND and the calibration resistor Rcal and the pump current source SQ and the reference current source SQr to a digital measuring system DMS, and wherein the first switching matrix provides at least the following switching connections:
Z_1: RGND with IPE and APE with SQ
Z_2: RGND with APE and IPE with SQ
Z_0: IPE with VM
Z_Rie: RE with SQr and IPE with VM.
Legal regulations prescribe the monitoring of the composition of the exhaust gas of internal combustion engines for compliance with limit values. For this purpose, in the exhaust gas, by means of regulated three-way catalytic converters, undesirable substances such as nitrogen oxides and carbon monoxide are converted into substances that can be regarded as noncritical, such as water vapor, carbon dioxide and nitrogen. This conversion presupposes that the air-fuel mixture fed to the internal combustion engine is in a specific composition range around a stoichiometric composition. The latter is designated by the parameter lambda=1. The composition of the air-fuel mixture is monitored by exhaust gas sensors provided in the exhaust gas duct of the internal combustion engines, for example in the form of broadband lambda probes, which determine the oxygen partial pressure. The correct function of the exhaust gas sensors, and in particular also the ageing resistance thereof, are greatly dependent on the electronic circuitry interconnection thereof. The function blocks of such a circuitry interconnection are described by way of example in the document DE 10 2006 061 565 A1.
The document DE 10 2008 001697 A1 in the name of the present applicant describes an improved circuitry interconnection that allows—in addition to the operation of the exhaust gas sensor—information about the operating state of the broadband lambda probe used there as exhaust gas sensor to be detected, stored and forwarded to a superordinate engine controller via a digital interface. This arrangement enables a diagnosis of the cable connections between the circuitry interconnection and the broadband lambda probe with respect to short circuit and interruption and also with respect to compliance with the voltages permissible at the terminals. The operational availability of the exhaust gas probe can be detected and the electrode polarization thereof and the ageing can be continuously monitored.
The as yet unpublished document R.330560 in the name of the present inventor describes a device for operating a broadband lambda probe in the exhaust gas duct of an internal combustion engine and for detecting information about the operating state of the broadband lambda probe, wherein the broadband lambda probe has as terminals a reference electrode terminal RE, an internal pump electrode terminal IPE, an external pump electrode terminal APE and a measurement terminal MES, wherein the device is connected to the terminals of the broadband lambda probe and a grounding resistor RGND and a calibration resistor Rcal, wherein the device has a first switching matrix for mutually connecting a pump current source SQ and a reference current source SQr to the terminals of the broadband lambda probe, the grounding resistor RGND, the calibration resistor Rcal and a reference voltage source URef, wherein the device has a second switching matrix for mutually connecting the terminals of the broadband lambda probe, the grounding resistor RGND and the calibration resistor Rcal and the pump current source SQ and the reference current source SQr to a digital measuring system DMS. In that case, provision is made for the first switching matrix to provide the following switching connections:
Z_Off: no switching connection
Z_G0: RGND to SQ
Z_GE: RGND to SQ and RE to SQ
Z_Gi: RGND to SQ and IPE to SQ
Z_Ga: RGND to SQ and APE to SQ
Z_Gi_ai: RGND to SQ, IPE to SQ and APE to SQr
Z_Gi_ei: RGND to SQ, IPE to SQ and RE to SQr
Z_K: RGND to MES and APE to SQ
Z_Ria: APE to SQr and IPE to VM
Z_Rie: RE to SQr and APE to VM
Z_0: IPE to VM
Z_Cal: Rcal to SQr
Z_1: RGND to IPE and APE to SQ
Z_2: RGND to APE and IPE to SQ.
Through corresponding circuitry interconnection of the first switching matrix and evaluation of the voltages established, in particular of the voltage drop across the grounding resistor RGND, various faults of the broadband lambda probe can be identified. For identifying faults, for example cable breaks in the terminals of the broadband lambda probe, special program sequences are provided in the control unit. These program sequences carry out measurements for identifying various faults of the exhaust gas probe in suitable operating modes of the broadband lambda probe, for example after switch-on (mode SWITCHON) or during the warm-up phase (mode WARMUP).